Add README.

Author: trekhleb

src/data-structures/stack/README.md

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+# Stack
+
+In computer science, a stack is an abstract data type that serves 
+as a collection of elements, with two principal operations:
+
+* **push**, which adds an element to the collection, and
+* **pop**, which removes the most recently added element that was not yet removed.
+
+The order in which elements come off a stack gives rise to its 
+alternative name, LIFO (last in, first out). Additionally, a 
+peek operation may give access to the top without modifying 
+the stack. The name "stack" for this type of structure comes 
+from the analogy to a set of physical items stacked on top of 
+each other, which makes it easy to take an item off the top 
+of the stack, while getting to an item deeper in the stack 
+may require taking off multiple other items first
+
+Simple representation of a stack runtime with push and pop operations.
+
+![Stack](https://upload.wikimedia.org/wikipedia/commons/b/b4/Lifo_stack.png)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Stack_(abstract_data_type))